Zirconium, hafnium, tantalum and niobium alkoxides (alcoholates) can be used for the deposition of corresponding metal oxide layers by means of Chemical Vapour Deposition (CVD) and are therefore valuable starting compounds for the production of extremely resistant components used e.g. in the electronics industry. Such metal oxide layers can also be produced from the corresponding zirconium, hafnium, tantalum or niobium alkoxides via hydrolysis by the sol-gel method. The very high dielectric constant makes it possible e.g. to use zirconium, hafnium and tantalum oxide layers in so-called DRAMs (Dynamic Random Access Read/Write Memories).
However, a problem in the electronics industry is the extreme demands made on the purity of the starting materials for such layers, i.e. the alkoxides, so there is no lack of e.g. descriptions in the patent literature for special processes for the purification of niobium and tantalum alkoxides.
The most common, technically simplest and most economic preparation of zirconium, hafnium, niobium and tantalum alkoxides is based on the corresponding metal chlorides and alcohols. A comprehensive survey is given in the book “Alkoxo and Aryloxo Derivatives of Metals” by D. C. Bradley, R. C. Mehrotra, I. P. Rothwell and A. Singh, Academic Press, 2001. A typical procedure is described e.g. in DE 10113169 A1.
Preparation from the metal chlorides inevitably produces chloride as one of the main impurities that have to be separated from the alkoxides. The Cl content of crude tantalum ethoxide prior to distillation is thus around 500-1000 ppm or more. For example, crude products prepared according to DE 10113169 A1 typically contain over 3000 ppm of Cl.
The removal of chloride is therefore also the most frequently cited subject of the invention in the aforementioned patent for the purification of Ta and Nb alkoxides. This is due in particular to the fact that distillation alone is a process of only limited suitability. For example, experience has shown that a simple high-vacuum distillation of crude tantalum ethoxide only reduces the Cl content to about half. Better results are achieved by distillation over packed columns. However, because of the high boiling point of most alkoxides, e.g. tantalum. ethoxide, even at low pressure, this method entails considerable expenditure in terms of time and energy and the technically expensive production of an operating pressure of <1 mbar. The separation effect of a single distillation is usually insufficient, so a rather uneconomic multiple column distillation is required. These difficulties also arise in the removal of Cl from zirconium and hafnium alkoxides.
The Applicants of JP 2002161059 A2 attempt to solve this problem by aftertreating the crude tantalum ethoxide (containing e.g. 450 ppm of Cl) with ethanolic alkali metal hydroxide solution, especially NaOH solution. Although this method reduces the Cl content to the desired low range, experience has shown that contact between the tantalum ethoxide and alkali metal compounds in such operations leads to unwanted high alkali metal contents in the product, despite distillation.
A similar procedure is proposed in JP 06220069 A2, which uses alkali metal hydrides (e.g. LiH) or complex compounds of these hydrides. This again gives rise to the problem of additional contamination of the tantalum alkoxides with alkali metal ions.
The method used in JP 06192148 A2 is no different. The alkali metal or alkaline earth metal alkoxides used here, e.g. lithium or sodium ethoxide, likewise reduce the Cl content in the desired manner, but once again there are unwanted high alkali metal ion concentrations in the product. For example, when sodium ethoxide is added to tantalum ethoxide, the Na value typically rises from <1 ppm to 2-4 ppm, despite subsequent distillation.
Finally, JP 10036299 A2 also indicates an aftertreatment with alkali metal or alkaline earth metal compounds, in this case using carbonates. The effects of the alkali metal or alkaline earth metal contaminations on the products substantially freed of Cl are once again disadvantageous, in the same sense as in the patent applications cited above. The silver carbonate also proposed in said patent application is disadvantageous on economic grounds alone.
In all the cited patent applications, basic alkali metal and alkaline earth metal compounds are used for the purification. This is obviously based on the fact that, in the technically conventional methods of preparing tantalum and niobium alkoxides, the use of ammonia as an auxiliary base for the reaction of metal pentahalides with alcohols does not afford or allow products containing less than e.g. 100 ppm of Cl without additional aftertreatment and purification steps, including expensive distillations. Thus the teaching of the state of the art is that the use of ammonia as auxiliary base in the reaction of tantalum or niobium chloride with alcohols gives crude products that always contain more than 100 ppm of Cl, and usually many times this amount, prior to further purification by distillation.